Review of Policy on MLS

Review of Policy on MLS

40TH ANNUAL CONFERENCE, Geneva, Switzerland, 19-23 March 2001

WP No. 83

Review of Policy on MLS


The IFATCA Policy on MLS was developed when ICAO planned to replace ILS with MLS as the world-wide standard precision approach aid. At the ICAO COM/OPS meeting in Montreal in 1995 this transition plan was reviewed and replaced with the present transition arrangements.

The new transition plan allows for the introduction of new Precision Approach aids as required in each ICAO region. In practice, there will not be a single transition from ILS to a new precision approach aid. Instead, new technology approach aids such as MLS and GNSS will be introduced according to local needs in each ICAO region. Even when new technology aids are installed, there will undoubtedly be a need for continued ILS operations to accommodate non- equipped aircraft. In some cases this will result in both ILS and MLS/GNSS being in use on the same runway for many years to come.

MLS has been installed at Amsterdam Schiphol and there are plans to introduce it at a number of other major European airports. ICAO is developing procedures for MLS operations.

Relevant IFATCA policy is included in sections 2.7 Microwave Landing System (MLS) and 2.8 Required Navigation Performance (RNP) for Approach and Landing.

This policy needs to be reviewed in the light of the new situation described above.


Any policy based on the old transition plan should be considered for deletion. Existing policy needs to be reviewed and additional policy written as appropriate. This paper addresses each of these items in turn.

Policy to be deleted

Policy statement 2.7 Microwave Landing System (MLS) was written before the ICAO COM/OPS meeting and addresses the old transition plan. It has now been superseded by the new transition arrangements and is therefore irrelevant.

A large part of policy statement 2.7 covers the subject of MLS Advanced Procedures. It is possible that this policy may be needed when these are introduced in the future. In order not to loose the policy on advanced procedures, it should be reviewed and relevant items retained. A separate working paper has been prepared with a review of Advanced Approach Procedures.

Review of policy on RNP

After the COM/OPS meeting, ICAO developed the concept of RNP for the Approach, Landing and Departure Phases of Flight to simplify the introduction of new technology approach and landing aids. Policy statement 2.8 Required Navigation Performance (RNP) for Approach and Landing takes this development into account. It is now appropriate to bring this policy up to date and include some of the latest developments.

With the publication of the ICAO Manual on RNP for the Approach, Landing and Departure Phases of Flight this principle has been established. The problem from the ATC point of view is that this is only a technical standard for the ground and airborne equipment. For ATC to benefit from the concept (e.g. to remove the need to ask each aircraft which approach aid is being used) ATC operational procedures based on RNP need to be developed.

At present there is a reluctance to do this as there is no general need for these procedures. It may be possible to establish some of these principles in the initial MLS procedures and build on them with other aids such as GNSS. Amendments are proposed to policy statement 2.8 to reflect this.

New policy

With the implementation of MLS and GNSS a mixed environment of ILS/MLS and GNSS will exist at most airports. This will inevitably increase the complexity of the ATC environment and an increase controller workload. It would appear that the best that can be done is to minimise the undesirable impact of these changes.

Development of the new procedures required is being undertaken by the Project Team on Low Visibility Procedures (PT/LVP) of the ICAO All Weather Operations Group (AWOG). This working paper is based on the progress of PT/LVP to date. Because of the ongoing nature of the work, it would seem best to develop policy based on what IFATCA would like to see as the long term outcome of this work.

PT/LVP has identified some principles for the development of MLS procedures:

All operations will be based on ILS look-alike straight in approaches. Advanced procedures will not be considered at this stage. In some cases, there may be a need for ATC to know which approach aid (e.g. ILS or MLS) each aircraft is using.

The most difficult situation to be faced is mixed mode ILS/MLS operations during Low Visibility Procedures (LVP). In this case there are potential safety risks if the spacing is reduced for MLS aircraft. Ideally, increased capacity would only be provided on a dedicated MLS runway but in practice there will be pressure to increase capacity in mixed mode particularly at single runway airports.

Other safety issues have been identified. There must be no confusion or misunderstanding over the approach aid being used. The point at which the pilot reports vacating the runway must be clear and correctly marked with the appropriate visual aids. It is possible that SMR will be required to operate these procedures.

In LVP, ATC will need to check which approach aid each aircraft will use and enter this into the ATC data system. The final approach spacing will be different for MLS aircraft. Aircraft need to be “Cleared MLS approach” or “Cleared ILS approach” as appropriate. These items will increase ATC workload. The point at which landing clearance is given to MLS aircraft will be different to that for ILS aircraft, creating yet more complications.

Aircraft equipage must be indicated on the Flight Plan and processed by CFMU. Separate flow rates may be declared for ILS and MLS aircraft.


The introduction of MLS and later GNSS will create a more complex ATC environment and mixed mode operations with ILS, MLS and GNSS, are inevitable. The desires of operators to see increase in capacity will put immense pressure on ATC to undertake this demanding task. SC1 should consider how these operations could be accommodated.

Outside LVP, ATC may not wish to know which approach aid is being used. Any unserviceabilities should be reported on the ATIS and pilots should select the appropriate approach aid. The only requirement for ATC will be to broadcast short notice failures to all aircraft.

Within LVP, the CFMU must process the MLS requirement in order to flow the correct mix of traffic according to the equipage. Ideally, aircraft would report the approach aid to be used by datalink, so avoiding the need for controllers to check this. In the meantime, this extra workload is inevitable. In the intermediate and final approach phases the introduction of “RNP style” procedures may avoid the need for controllers to refer to the approach aid being used.

In ILS/MLS mixed mode operations, any problems with the visual aids and the use of SMR must be resolved and a safety assessment undertaken.

Existing policy on MLS should be deleted and policy relevant to the new transition plan should be adopted. The current policy on RNP should be updated in the light of the new situation.

It is recommended that:

Required Navigation Performance (RNP) for the Approach, Landing and Departure phases of Flight

The concept of Required Navigation Performance (RNP) is to determine the accuracy of the aircraft navigation system. The fundamental feature is that it only defines a standard of equipment performance therefore it is of no interest to controller how the navigation is achieved but only that the aircraft is equipped to the standard required in a particular piece of airspace. This principle offers considerable advantages to ATC when translated into operational procedures that can be used by pilots and controllers.

IFATCA policy is:

“RNP based operational procedures should be developed in parallel with new approach aids such as MLS and GNSS and should, ideally, be introduced at the same time as these new aids. After the introduction of a new aid, the full benefits of RNP procedures may not be achieved while ILS is still in use, therefore the ILS should be withdrawn as soon as practical.

A significant factor in the determination of RNP procedures should be the aim of limiting the increase in ATC workload due to the proliferation in landing aids expected in the future .”

New Technology Approach and Landing Aids

ICAO has developed a transition plan allowing the introduction of new technology approach and landing aids (currently MLS and GNSS). IFATCA is concerned that the more complex environment created by the use of a number of new approach aids and by the continued use of ILS will create operational problems for controllers.

These problems can be reduced by using the principles associated with RNP in the design of operational procedures, so avoiding the need for the controller to know which aircraft is using which precision approach aid and reducing RTF.

IFATCA policy is:

“Operational procedures for New Technology Approach and Landing Aids should be based on the principles associated with RNP.

A safety assessment should be carried out before any mixed mode operations or reductions in spacing are undertaken and any remedial measures identified must be carried out.

Ground protection and associated visual aids must be provided for the most demanding approach aid.

When SMR is required in operational procedures, the SMR must be serviceable and displayed to the controller during these operations. ( Note: See SMR Policy)

To permit advanced planning the approach aid capability of the aircraft should be included in the flightplan and displayed to the controller using the appropriate data display system.

In busy terminal airspace datalink should be introduced to enable aircraft to report their approach capability without increasing controller workload.

Where New Technology Approach and Landing Aids are introduced, any additional staff training and equipment to integrate this traffic into the ATC system must be provided before these operations commence .”

Last Update: September 29, 2020  

March 12, 2020   681   Jean-Francois Lepage    2001    

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